It is well known that the addition of water, or other suitable liquids or mixtures of liquids, into the working medium of gas turbines can be used for increasing the power that can be generated by gas turbine units. The increase of available power is due to, among other reasons, the cooling effect of the added water reducing the compressor power consumption and allowing an increased firing of the gas turbine unit, as well as due to the increased mass flow passing the turbine blades.
Water can be added either in the form of vapor, that is in the form of humidified air or, and this relates to the present invention, in the form of small liquid droplets, i.e. water can be added above the saturation level of water in the air. This technique, known as over-fogging, is usually carried out by adding liquid droplets of particular size to the air stream entering the compressor of the gas turbine unit (so-called ‘wet compression’). ‘Wet compression’ allows to increase the power available due to the reduction of work required for compression of the inlet air, as the latent heat for evaporation of this water cools the inlet air stream when it passes the compressor stages.
There are a number of documents describing particular designs of gas turbine units and methods for their operation using ‘wet compression’. For example U.S. Pat. No. 5,930,990 as well as its continuation-in-part U.S. Pat. No. 5,867,977 describe an apparatus for ‘wet compression’ as well as a method for increasing the power available by using ‘wet compression’. These documents concern gas turbine units with one single combustion chamber, where small droplets of water are being added to the intake air entering the compressor. The increase of added water is carried out in a stepwise, incremental manner, and due to problems of thermal non-equilibrium induced in the components of the gas turbine unit downstream of the injection (thermal shock leading to cracking in certain alloyed components and to undesired reduction of clearance between parts which are in motion relative to each other during operation), this stepwise addition of water is proposed to be carried out on a timescale of 10-20 minutes. Additionally the proposed means for adding the water are designed like a grid of nozzles lying in one plane and being arranged substantially perpendicular to the direction of flow of the intake air stream. This stepwise increase of water added to the intake air can, according to this document, be achieved by either increasingly feeding the nozzles with water or by systematically feeding more and more nozzles with water (sequential increase in time or in position).
In order to control and to optimize the addition of water when using wet compression, WO 00/50739 proposes a particular apparatus for monitoring destructive ‘wet compression’ power augmentation-related casing distortions in gas turbine units. Also this document points out that in order to avoid thermal stress when activating ‘wet compression’, the addition of water has to be carried out smoothly under a careful monitoring of the associated distortions of the casing and other components of the gas turbine unit.
Another particular device to be used for ‘wet compression’ is proposed in U.S. Pat. No. 6,216,443. The water is added by means of a liquid droplet injection device provided on the upstream side of a compressor, and on the downstream side of a silencer. The addition of water is controlled by means of a valve, and it is also pointed out that due to thermal stress and due to the fact that concomitantly other parameters have to be adjusted when adding water to the intake air (for example the addition of fuel has to be adjusted), the addition has to be performed sufficiently slowly.